2 * sigreturn.c - tests for x86 sigreturn(2) and exit-to-userspace
3 * Copyright (c) 2014-2015 Andrew Lutomirski
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms and conditions of the GNU General Public License,
7 * version 2, as published by the Free Software Foundation.
9 * This program is distributed in the hope it will be useful, but
10 * WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 * General Public License for more details.
14 * This is a series of tests that exercises the sigreturn(2) syscall and
15 * the IRET / SYSRET paths in the kernel.
17 * For now, this focuses on the effects of unusual CS and SS values,
18 * and it has a bunch of tests to make sure that ESP/RSP is restored
21 * The basic idea behind these tests is to raise(SIGUSR1) to create a
22 * sigcontext frame, plug in the values to be tested, and then return,
23 * which implicitly invokes sigreturn(2) and programs the user context
26 * For tests for which we expect sigreturn and the subsequent return to
27 * user mode to succeed, we return to a short trampoline that generates
28 * SIGTRAP so that the meat of the tests can be ordinary C code in a
31 * The inner workings of each test is documented below.
33 * Do not run on outdated, unpatched kernels at risk of nasty crashes.
41 #include <sys/syscall.h>
47 #include <sys/signal.h>
48 #include <sys/ucontext.h>
54 #include <sys/ptrace.h>
57 /* Pull in AR_xyz defines. */
58 typedef unsigned int u32;
59 typedef unsigned short u16;
60 #include "../../../../arch/x86/include/asm/desc_defs.h"
63 * Copied from asm/ucontext.h, as asm/ucontext.h conflicts badly with the glibc
68 * UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on
69 * kernels that save SS in the sigcontext. All kernels that set
70 * UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp
71 * regardless of SS (i.e. they implement espfix).
73 * Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS
74 * when delivering a signal that came from 64-bit code.
76 * Sigreturn restores SS as follows:
78 * if (saved SS is valid || UC_STRICT_RESTORE_SS is set ||
79 * saved CS is not 64-bit)
80 * new SS = saved SS (will fail IRET and signal if invalid)
82 * new SS = a flat 32-bit data segment
84 #define UC_SIGCONTEXT_SS 0x2
85 #define UC_STRICT_RESTORE_SS 0x4
89 * In principle, this test can run on Linux emulation layers (e.g.
90 * Illumos "LX branded zones"). Solaris-based kernels reserve LDT
91 * entries 0-5 for their own internal purposes, so start our LDT
92 * allocations above that reservation. (The tests don't pass on LX
93 * branded zones, but at least this lets them run.)
97 /* An aligned stack accessible through some of our segments. */
98 static unsigned char stack16[65536] __attribute__((aligned(4096)));
101 * An aligned int3 instruction used as a trampoline. Some of the tests
102 * want to fish out their ss values, so this trampoline copies ss to eax
105 asm (".pushsection .text\n\t"
106 ".type int3, @function\n\t"
111 ".size int3, . - int3\n\t"
112 ".align 4096, 0xcc\n\t"
114 extern char int3[4096];
117 * At startup, we prepapre:
119 * - ldt_nonexistent_sel: An LDT entry that doesn't exist (all-zero
120 * descriptor or out of bounds).
121 * - code16_sel: A 16-bit LDT code segment pointing to int3.
122 * - data16_sel: A 16-bit LDT data segment pointing to stack16.
123 * - npcode32_sel: A 32-bit not-present LDT code segment pointing to int3.
124 * - npdata32_sel: A 32-bit not-present LDT data segment pointing to stack16.
125 * - gdt_data16_idx: A 16-bit GDT data segment pointing to stack16.
126 * - gdt_npdata32_idx: A 32-bit not-present GDT data segment pointing to
129 * For no particularly good reason, xyz_sel is a selector value with the
130 * RPL and LDT bits filled in, whereas xyz_idx is just an index into the
131 * descriptor table. These variables will be zero if their respective
132 * segments could not be allocated.
134 static unsigned short ldt_nonexistent_sel;
135 static unsigned short code16_sel, data16_sel, npcode32_sel, npdata32_sel;
137 static unsigned short gdt_data16_idx, gdt_npdata32_idx;
139 static unsigned short GDT3(int idx)
141 return (idx << 3) | 3;
144 static unsigned short LDT3(int idx)
146 return (idx << 3) | 7;
149 /* Our sigaltstack scratch space. */
150 static char altstack_data[SIGSTKSZ];
152 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
156 memset(&sa, 0, sizeof(sa));
157 sa.sa_sigaction = handler;
158 sa.sa_flags = SA_SIGINFO | flags;
159 sigemptyset(&sa.sa_mask);
160 if (sigaction(sig, &sa, 0))
164 static void clearhandler(int sig)
167 memset(&sa, 0, sizeof(sa));
168 sa.sa_handler = SIG_DFL;
169 sigemptyset(&sa.sa_mask);
170 if (sigaction(sig, &sa, 0))
174 static void add_ldt(const struct user_desc *desc, unsigned short *var,
177 if (syscall(SYS_modify_ldt, 1, desc, sizeof(*desc)) == 0) {
178 *var = LDT3(desc->entry_number);
180 printf("[NOTE]\tFailed to create %s segment\n", name);
185 static void setup_ldt(void)
187 if ((unsigned long)stack16 > (1ULL << 32) - sizeof(stack16))
188 errx(1, "stack16 is too high\n");
189 if ((unsigned long)int3 > (1ULL << 32) - sizeof(int3))
190 errx(1, "int3 is too high\n");
192 ldt_nonexistent_sel = LDT3(LDT_OFFSET + 2);
194 const struct user_desc code16_desc = {
195 .entry_number = LDT_OFFSET + 0,
196 .base_addr = (unsigned long)int3,
199 .contents = 2, /* Code, not conforming */
202 .seg_not_present = 0,
205 add_ldt(&code16_desc, &code16_sel, "code16");
207 const struct user_desc data16_desc = {
208 .entry_number = LDT_OFFSET + 1,
209 .base_addr = (unsigned long)stack16,
212 .contents = 0, /* Data, grow-up */
215 .seg_not_present = 0,
218 add_ldt(&data16_desc, &data16_sel, "data16");
220 const struct user_desc npcode32_desc = {
221 .entry_number = LDT_OFFSET + 3,
222 .base_addr = (unsigned long)int3,
225 .contents = 2, /* Code, not conforming */
228 .seg_not_present = 1,
231 add_ldt(&npcode32_desc, &npcode32_sel, "npcode32");
233 const struct user_desc npdata32_desc = {
234 .entry_number = LDT_OFFSET + 4,
235 .base_addr = (unsigned long)stack16,
238 .contents = 0, /* Data, grow-up */
241 .seg_not_present = 1,
244 add_ldt(&npdata32_desc, &npdata32_sel, "npdata32");
246 struct user_desc gdt_data16_desc = {
248 .base_addr = (unsigned long)stack16,
251 .contents = 0, /* Data, grow-up */
254 .seg_not_present = 0,
258 if (syscall(SYS_set_thread_area, &gdt_data16_desc) == 0) {
260 * This probably indicates vulnerability to CVE-2014-8133.
261 * Merely getting here isn't definitive, though, and we'll
262 * diagnose the problem for real later on.
264 printf("[WARN]\tset_thread_area allocated data16 at index %d\n",
265 gdt_data16_desc.entry_number);
266 gdt_data16_idx = gdt_data16_desc.entry_number;
268 printf("[OK]\tset_thread_area refused 16-bit data\n");
271 struct user_desc gdt_npdata32_desc = {
273 .base_addr = (unsigned long)stack16,
276 .contents = 0, /* Data, grow-up */
279 .seg_not_present = 1,
283 if (syscall(SYS_set_thread_area, &gdt_npdata32_desc) == 0) {
285 * As a hardening measure, newer kernels don't allow this.
287 printf("[WARN]\tset_thread_area allocated npdata32 at index %d\n",
288 gdt_npdata32_desc.entry_number);
289 gdt_npdata32_idx = gdt_npdata32_desc.entry_number;
291 printf("[OK]\tset_thread_area refused 16-bit data\n");
295 /* State used by our signal handlers. */
296 static gregset_t initial_regs, requested_regs, resulting_regs;
298 /* Instructions for the SIGUSR1 handler. */
299 static volatile unsigned short sig_cs, sig_ss;
300 static volatile sig_atomic_t sig_trapped, sig_err, sig_trapno;
302 static volatile sig_atomic_t sig_corrupt_final_ss;
305 /* Abstractions for some 32-bit vs 64-bit differences. */
307 # define REG_IP REG_RIP
308 # define REG_SP REG_RSP
309 # define REG_CX REG_RCX
312 unsigned short cs, gs, fs, ss;
315 static unsigned short *ssptr(ucontext_t *ctx)
317 struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS];
321 static unsigned short *csptr(ucontext_t *ctx)
323 struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS];
327 # define REG_IP REG_EIP
328 # define REG_SP REG_ESP
329 # define REG_CX REG_ECX
331 static greg_t *ssptr(ucontext_t *ctx)
333 return &ctx->uc_mcontext.gregs[REG_SS];
336 static greg_t *csptr(ucontext_t *ctx)
338 return &ctx->uc_mcontext.gregs[REG_CS];
343 * Checks a given selector for its code bitness or returns -1 if it's not
344 * a usable code segment selector.
346 int cs_bitness(unsigned short cs)
348 uint32_t valid = 0, ar;
349 asm ("lar %[cs], %[ar]\n\t"
351 "mov $1, %[valid]\n\t"
353 : [ar] "=r" (ar), [valid] "+rm" (valid)
359 bool db = (ar & (1 << 22));
360 bool l = (ar & (1 << 21));
363 return -1; /* Not code. */
372 return -1; /* Unknown bitness. */
376 * Checks a given selector for its code bitness or returns -1 if it's not
377 * a usable code segment selector.
379 bool is_valid_ss(unsigned short cs)
381 uint32_t valid = 0, ar;
382 asm ("lar %[cs], %[ar]\n\t"
384 "mov $1, %[valid]\n\t"
386 : [ar] "=r" (ar), [valid] "+rm" (valid)
392 if ((ar & AR_TYPE_MASK) != AR_TYPE_RWDATA &&
393 (ar & AR_TYPE_MASK) != AR_TYPE_RWDATA_EXPDOWN)
399 /* Number of errors in the current test case. */
400 static volatile sig_atomic_t nerrs;
402 static void validate_signal_ss(int sig, ucontext_t *ctx)
405 bool was_64bit = (cs_bitness(*csptr(ctx)) == 64);
407 if (!(ctx->uc_flags & UC_SIGCONTEXT_SS)) {
408 printf("[FAIL]\tUC_SIGCONTEXT_SS was not set\n");
412 * This happens on Linux 4.1. The rest will fail, too, so
413 * return now to reduce the noise.
418 /* UC_STRICT_RESTORE_SS is set iff we came from 64-bit mode. */
419 if (!!(ctx->uc_flags & UC_STRICT_RESTORE_SS) != was_64bit) {
420 printf("[FAIL]\tUC_STRICT_RESTORE_SS was wrong in signal %d\n",
425 if (is_valid_ss(*ssptr(ctx))) {
427 * DOSEMU was written before 64-bit sigcontext had SS, and
428 * it tries to figure out the signal source SS by looking at
429 * the physical register. Make sure that keeps working.
431 unsigned short hw_ss;
432 asm ("mov %%ss, %0" : "=rm" (hw_ss));
433 if (hw_ss != *ssptr(ctx)) {
434 printf("[FAIL]\tHW SS didn't match saved SS\n");
442 * SIGUSR1 handler. Sets CS and SS as requested and points IP to the
443 * int3 trampoline. Sets SP to a large known value so that we can see
444 * whether the value round-trips back to user mode correctly.
446 static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
448 ucontext_t *ctx = (ucontext_t*)ctx_void;
450 validate_signal_ss(sig, ctx);
452 memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
454 *csptr(ctx) = sig_cs;
455 *ssptr(ctx) = sig_ss;
457 ctx->uc_mcontext.gregs[REG_IP] =
458 sig_cs == code16_sel ? 0 : (unsigned long)&int3;
459 ctx->uc_mcontext.gregs[REG_SP] = (unsigned long)0x8badf00d5aadc0deULL;
460 ctx->uc_mcontext.gregs[REG_CX] = 0;
462 memcpy(&requested_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
463 requested_regs[REG_CX] = *ssptr(ctx); /* The asm code does this. */
469 * Called after a successful sigreturn (via int3) or from a failed
470 * sigreturn (directly by kernel). Restores our state so that the
471 * original raise(SIGUSR1) returns.
473 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
475 ucontext_t *ctx = (ucontext_t*)ctx_void;
477 validate_signal_ss(sig, ctx);
479 sig_err = ctx->uc_mcontext.gregs[REG_ERR];
480 sig_trapno = ctx->uc_mcontext.gregs[REG_TRAPNO];
483 asm ("mov %%ss,%0" : "=r" (ss));
485 greg_t asm_ss = ctx->uc_mcontext.gregs[REG_CX];
486 if (asm_ss != sig_ss && sig == SIGTRAP) {
487 /* Sanity check failure. */
488 printf("[FAIL]\tSIGTRAP: ss = %hx, frame ss = %hx, ax = %llx\n",
489 ss, *ssptr(ctx), (unsigned long long)asm_ss);
493 memcpy(&resulting_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
494 memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));
497 if (sig_corrupt_final_ss) {
498 if (ctx->uc_flags & UC_STRICT_RESTORE_SS) {
499 printf("[FAIL]\tUC_STRICT_RESTORE_SS was set inappropriately\n");
503 * DOSEMU transitions from 32-bit to 64-bit mode by
504 * adjusting sigcontext, and it requires that this work
505 * even if the saved SS is bogus.
507 printf("\tCorrupting SS on return to 64-bit mode\n");
517 /* Tests recovery if !UC_STRICT_RESTORE_SS */
518 static void sigusr2(int sig, siginfo_t *info, void *ctx_void)
520 ucontext_t *ctx = (ucontext_t*)ctx_void;
522 if (!(ctx->uc_flags & UC_STRICT_RESTORE_SS)) {
523 printf("[FAIL]\traise(2) didn't set UC_STRICT_RESTORE_SS\n");
525 return; /* We can't do the rest. */
528 ctx->uc_flags &= ~UC_STRICT_RESTORE_SS;
531 /* Return. The kernel should recover without sending another signal. */
534 static int test_nonstrict_ss(void)
536 clearhandler(SIGUSR1);
537 clearhandler(SIGTRAP);
538 clearhandler(SIGSEGV);
539 clearhandler(SIGILL);
540 sethandler(SIGUSR2, sigusr2, 0);
544 printf("[RUN]\tClear UC_STRICT_RESTORE_SS and corrupt SS\n");
547 printf("[OK]\tIt worked\n");
553 /* Finds a usable code segment of the requested bitness. */
554 int find_cs(int bitness)
556 unsigned short my_cs;
558 asm ("mov %%cs,%0" : "=r" (my_cs));
560 if (cs_bitness(my_cs) == bitness)
562 if (cs_bitness(my_cs + (2 << 3)) == bitness)
563 return my_cs + (2 << 3);
564 if (my_cs > (2<<3) && cs_bitness(my_cs - (2 << 3)) == bitness)
565 return my_cs - (2 << 3);
566 if (cs_bitness(code16_sel) == bitness)
569 printf("[WARN]\tCould not find %d-bit CS\n", bitness);
573 static int test_valid_sigreturn(int cs_bits, bool use_16bit_ss, int force_ss)
575 int cs = find_cs(cs_bits);
577 printf("[SKIP]\tCode segment unavailable for %d-bit CS, %d-bit SS\n",
578 cs_bits, use_16bit_ss ? 16 : 32);
582 if (force_ss != -1) {
587 printf("[SKIP]\tData segment unavailable for %d-bit CS, 16-bit SS\n",
593 asm volatile ("mov %%ss,%0" : "=r" (sig_ss));
599 printf("[RUN]\tValid sigreturn: %d-bit CS (%hx), %d-bit SS (%hx%s)\n",
600 cs_bits, sig_cs, use_16bit_ss ? 16 : 32, sig_ss,
601 (sig_ss & 4) ? "" : ", GDT");
608 * Check that each register had an acceptable value when the
609 * int3 trampoline was invoked.
611 for (int i = 0; i < NGREG; i++) {
612 greg_t req = requested_regs[i], res = resulting_regs[i];
614 if (i == REG_TRAPNO || i == REG_IP)
615 continue; /* don't care */
619 * If we were using a 16-bit stack segment, then
620 * the kernel is a bit stuck: IRET only restores
621 * the low 16 bits of ESP/RSP if SS is 16-bit.
622 * The kernel uses a hack to restore bits 31:16,
623 * but that hack doesn't help with bits 63:32.
624 * On Intel CPUs, bits 63:32 end up zeroed, and, on
625 * AMD CPUs, they leak the high bits of the kernel
626 * espfix64 stack pointer. There's very little that
627 * the kernel can do about it.
629 * Similarly, if we are returning to a 32-bit context,
630 * the CPU will often lose the high 32 bits of RSP.
636 if (cs_bits != 64 && ((res ^ req) & 0xFFFFFFFF) == 0) {
637 printf("[NOTE]\tSP: %llx -> %llx\n",
638 (unsigned long long)req,
639 (unsigned long long)res);
643 printf("[FAIL]\tSP mismatch: requested 0x%llx; got 0x%llx\n",
644 (unsigned long long)requested_regs[i],
645 (unsigned long long)resulting_regs[i]);
650 bool ignore_reg = false;
655 if (i == REG_CSGSFS) {
656 struct selectors *req_sels =
657 (void *)&requested_regs[REG_CSGSFS];
658 struct selectors *res_sels =
659 (void *)&resulting_regs[REG_CSGSFS];
660 if (req_sels->cs != res_sels->cs) {
661 printf("[FAIL]\tCS mismatch: requested 0x%hx; got 0x%hx\n",
662 req_sels->cs, res_sels->cs);
666 if (req_sels->ss != res_sels->ss) {
667 printf("[FAIL]\tSS mismatch: requested 0x%hx; got 0x%hx\n",
668 req_sels->ss, res_sels->ss);
676 /* Sanity check on the kernel */
677 if (i == REG_CX && req != res) {
678 printf("[FAIL]\tCX (saved SP) mismatch: requested 0x%llx; got 0x%llx\n",
679 (unsigned long long)req,
680 (unsigned long long)res);
685 if (req != res && !ignore_reg) {
686 printf("[FAIL]\tReg %d mismatch: requested 0x%llx; got 0x%llx\n",
687 i, (unsigned long long)req,
688 (unsigned long long)res);
694 printf("[OK]\tall registers okay\n");
699 static int test_bad_iret(int cs_bits, unsigned short ss, int force_cs)
701 int cs = force_cs == -1 ? find_cs(cs_bits) : force_cs;
708 printf("[RUN]\t%d-bit CS (%hx), bogus SS (%hx)\n",
709 cs_bits, sig_cs, sig_ss);
714 char errdesc[32] = "";
716 const char *src = (sig_err & 1) ? " EXT" : "";
718 if ((sig_err & 0x6) == 0x0)
720 else if ((sig_err & 0x6) == 0x4)
722 else if ((sig_err & 0x6) == 0x2)
727 sprintf(errdesc, "%s%s index %d, ",
728 table, src, sig_err >> 3);
732 if (sig_trapno == 13)
733 strcpy(trapname, "GP");
734 else if (sig_trapno == 11)
735 strcpy(trapname, "NP");
736 else if (sig_trapno == 12)
737 strcpy(trapname, "SS");
738 else if (sig_trapno == 32)
739 strcpy(trapname, "IRET"); /* X86_TRAP_IRET */
741 sprintf(trapname, "%d", sig_trapno);
743 printf("[OK]\tGot #%s(0x%lx) (i.e. %s%s)\n",
744 trapname, (unsigned long)sig_err,
745 errdesc, strsignal(sig_trapped));
749 * This also implicitly tests UC_STRICT_RESTORE_SS:
750 * We check that these signals set UC_STRICT_RESTORE_SS and,
751 * if UC_STRICT_RESTORE_SS doesn't cause strict behavior,
752 * then we won't get SIGSEGV.
754 printf("[FAIL]\tDid not get SIGSEGV\n");
762 unsigned short my_cs, my_ss;
764 asm volatile ("mov %%cs,%0" : "=r" (my_cs));
765 asm volatile ("mov %%ss,%0" : "=r" (my_ss));
769 .ss_sp = altstack_data,
772 if (sigaltstack(&stack, NULL) != 0)
773 err(1, "sigaltstack");
775 sethandler(SIGUSR1, sigusr1, 0);
776 sethandler(SIGTRAP, sigtrap, SA_ONSTACK);
778 /* Easy cases: return to a 32-bit SS in each possible CS bitness. */
779 total_nerrs += test_valid_sigreturn(64, false, -1);
780 total_nerrs += test_valid_sigreturn(32, false, -1);
781 total_nerrs += test_valid_sigreturn(16, false, -1);
784 * Test easy espfix cases: return to a 16-bit LDT SS in each possible
785 * CS bitness. NB: with a long mode CS, the SS bitness is irrelevant.
787 * This catches the original missing-espfix-on-64-bit-kernels issue
788 * as well as CVE-2014-8134.
790 total_nerrs += test_valid_sigreturn(64, true, -1);
791 total_nerrs += test_valid_sigreturn(32, true, -1);
792 total_nerrs += test_valid_sigreturn(16, true, -1);
794 if (gdt_data16_idx) {
796 * For performance reasons, Linux skips espfix if SS points
797 * to the GDT. If we were able to allocate a 16-bit SS in
798 * the GDT, see if it leaks parts of the kernel stack pointer.
800 * This tests for CVE-2014-8133.
802 total_nerrs += test_valid_sigreturn(64, true,
803 GDT3(gdt_data16_idx));
804 total_nerrs += test_valid_sigreturn(32, true,
805 GDT3(gdt_data16_idx));
806 total_nerrs += test_valid_sigreturn(16, true,
807 GDT3(gdt_data16_idx));
811 /* Nasty ABI case: check SS corruption handling. */
812 sig_corrupt_final_ss = 1;
813 total_nerrs += test_valid_sigreturn(32, false, -1);
814 total_nerrs += test_valid_sigreturn(32, true, -1);
815 sig_corrupt_final_ss = 0;
819 * We're done testing valid sigreturn cases. Now we test states
820 * for which sigreturn itself will succeed but the subsequent
821 * entry to user mode will fail.
823 * Depending on the failure mode and the kernel bitness, these
824 * entry failures can generate SIGSEGV, SIGBUS, or SIGILL.
826 clearhandler(SIGTRAP);
827 sethandler(SIGSEGV, sigtrap, SA_ONSTACK);
828 sethandler(SIGBUS, sigtrap, SA_ONSTACK);
829 sethandler(SIGILL, sigtrap, SA_ONSTACK); /* 32-bit kernels do this */
831 /* Easy failures: invalid SS, resulting in #GP(0) */
832 test_bad_iret(64, ldt_nonexistent_sel, -1);
833 test_bad_iret(32, ldt_nonexistent_sel, -1);
834 test_bad_iret(16, ldt_nonexistent_sel, -1);
836 /* These fail because SS isn't a data segment, resulting in #GP(SS) */
837 test_bad_iret(64, my_cs, -1);
838 test_bad_iret(32, my_cs, -1);
839 test_bad_iret(16, my_cs, -1);
841 /* Try to return to a not-present code segment, triggering #NP(SS). */
842 test_bad_iret(32, my_ss, npcode32_sel);
845 * Try to return to a not-present but otherwise valid data segment.
846 * This will cause IRET to fail with #SS on the espfix stack. This
847 * exercises CVE-2014-9322.
849 * Note that, if espfix is enabled, 64-bit Linux will lose track
850 * of the actual cause of failure and report #GP(0) instead.
851 * This would be very difficult for Linux to avoid, because
852 * espfix64 causes IRET failures to be promoted to #DF, so the
853 * original exception frame is never pushed onto the stack.
855 test_bad_iret(32, npdata32_sel, -1);
858 * Try to return to a not-present but otherwise valid data
859 * segment without invoking espfix. Newer kernels don't allow
860 * this to happen in the first place. On older kernels, though,
861 * this can trigger CVE-2014-9322.
863 if (gdt_npdata32_idx)
864 test_bad_iret(32, GDT3(gdt_npdata32_idx), -1);
867 total_nerrs += test_nonstrict_ss();
870 return total_nerrs ? 1 : 0;